Carbon nanotubes (CNTs) may be used in a variety of devices, and have demonstrated outstanding absorption of light in a wide wavelength range, making them promising candidates for opto-electronic detector applications. Such photoresponse under infrared (IR) radiation may be a dominantly exciton (binded electron-hole pair) generation and dissociation either into the phonons (or lattice vibration manifested as sample temperature rise), or into free photo-generated charge carriers (photo-carriers). The exciton mechanism is in contrast to the direct photoconductivity possibly due to transition associated to the series of van Hove singularity in one-dimensional electronic density states.
With single wall CNTs (SWCNT), a one-dimensional system, the binding energy of the exciton is high (on the order of a few hundreds of milli-electron-volt), leading to difficulties in dissociation of the photo-excited excitons to photo-carriers without a provided strong electric field. Additionally, bolometric response may not be observable if the SWCNT's thermal link to the environment is sufficiently high. This is particularly true for SWCNTs, which have a high thermal conductance of up to 8000 W/mK at room temperature. Reducing the SWCNT film's thermal link to the environment is thus necessary in order to obtain an adequate bolometric photoresponse.
One way to reduce the thermal link is to suspend SWCNT films, which may enhance the bolometric photoresponse. Nevertheless, the best-obtained uncooled figure-of-merit detectivity (D*) around 4.5×105 cmHz1/2/W is nearly three orders of magnitude lower than that of conventional VOx bolometers. In addition, the response time of the SWCNT-film bolometers, in the range of 40-60 ms occurs in circumstances when SWCNTs are suspended, and must be further reduced by at least an order of magnitude for practical IR imaging systems. Further improvement of SWCNT-bolometer performance faces several fundamental limitations associated with the SWCNT films including reduced light absorption at small thicknesses approaching the CNT percolation thresholds, difficulties in suspending very thin CNT films, a large number of inter-tube junctions that may limit electrical and thermal transport, and a large surface area that amplifies effects of molecules such as oxygen absorbed/attached to the SWCNT surface. SWCNT have only one CNT shell so the surface effect, which is affected by almost any species absorbed/attached of the CNT surface, dominates the device properties. Consequently, the applications of SWCNTs on opto-electronic detections may be limited.
Individual CNTs may also be used to build Schottky devices, which typically consist a Schottky contact paired with another Ohmic contact on the other side, by collecting the photocurrent induced by the incident photons. The photocurrent is primarily generated from the excitons separated by the build-in electrical potential at the metal-CNT Schottky interface. However, the best current responsivity is below a few tens of A/W for CNT Schottky detectors and photocurrent may not be efficiently collected. The performances of previously reported CNT Schottky devices are far below practical requirements